1. Field of the Invention
This invention relates to a near-field scanning optical microscope in which a probe is placed closed to the surface of a specimen and while the surface of the specimen is scanned with the probe, a region proximate to the surface of the specimen is measured with light.
2. Description of Related Art
A near-field scanning optical microscope (which is hereinafter abbreviated to NSOM), in contrast with an optical microscope whose resolving power is restricted by a diffraction limit, is designed so that a probe whose aperture or radius of curvature of the tip is smaller than the wavelength of light used for measurement is placed close to the surface of a specimen, which is scanned with the probe to measure optical properties in a minute area of the specimen. This NSOM is such that, for example, in an aperture probe, if the aperture diameter of its tip is 50 nm, the resolving power corresponds to about 50 nm, while in a scattering probe, the resolving power corresponds to the order of the radius of curvature of the probe tip (several tens of nanometers). In this way, the NSOM brings about the resolving power corresponding to the size of the outside diameter of the probe tip (several tens of nanometers). Thus, the application of the NSOM to the field of industry or medicine is particularly expected.
NSOMs are available in a variety of structural types. For example, in view of techniques of acquiring optical information, there are structures in which illumination light is rendered incident on the back surface of the specimen so that evanescent light produced above the front surface (which is hereinafter referred to simply as the surface) of the specimen is introduced through the probe; those in which illumination light is rendered incident on the surface of the specimen so that scattered light produced from the specimen is introduced into the probe with a minute aperture; and those in which illumination light is emitted from the probe with a minute aperture toward the specimen so that transmitted light or scattered light of the specimen is introduced.
The measurement of a distance between the surface of the specimen and the probe where a region proximate to the surface of the specimen is scanned, involves the use of a technique of utilizing intensity attenuation characteristics of evanescent light in a traveling direction of the evanescent light and a perpendicular direction to detect the evanescent light produced on the surface side of the specimen by rendering illumination light incident on the back surface of the specimen, or a technique of utilizing an atomic force microscope (AFM) to detect the displacement of the probe caused by a force exerted between the specimen and the probe.
In each of the NSOMs having the mechanisms mentioned above, the distance between the surface of the specimen and the probe, the position of the probe, or the intensity of light detected is held constant, and at the same time, the region proximate to the surface of the specimen is scanned. In this way, the NSOM is capable of measuring the surface profile of the specimen and a difference in an optical constant (absorptance, refractive index, etc.) to an order that cannot be done with an ordinary optical microscope.
For the measurement of a device, which diminishes in size, utilizing light (for example, an integrated optical device, such as a photoswitch, which has been considerably studied in recent years or a device having an angular component parameter such as a directivity pattern of radiation intensity of a minute emitting element), three-dimensional information on the surface of the device is required. A conventional NSOM, however, is capable of measuring nothing but a two-dimensional plane nearly parallel to the surface of the specimen in the region proximate to the specimen. Thus, for such a specimen in which the conventional NSOM is required to measure the three-dimensional information it becomes necessary for the position of the probe for scanning be shifted to make measurements many times and often. Moreover, the results of individual measurements must be synthesized for interpretation.
As mentioned above, whenever the position of the probe is shifted, the measurement must be done, and thus the conventional apparatus encounters the problem that unnecessary time is required for measurement. Furthermore, where for measurements such a material that properties of the specimen, including its profile, change with time (namely, the profile is always unstable), there is the drawback that the relation between data derived from individual measurements ceases to be maintainable.